Light olefins, particularly ethylene and propylene, are important intermediates in the manufacture of a variety of chemical products. The limited availability and high cost of petroleum sources has caused an increase in the cost of producing light olefins from such sources. Together with geographic differences in availability and rapid petrochemical growth in developing economies, these factors are promoting a search for alternative materials for light-olefin production. Oxygenates such as alcohols, more particularly methanol and ethanol, may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics, municipal wastes, or other organic materials. Thus, alcohols provide alternative routes for the production of olefins and derivatives.
The conversion of methanol to yield light olefins is well known. “Hydrocarbons from Methanol” by Clarence D. Chang, published by Marcel Dekker, Inc. N.Y. (1983) presents a survey and summary of the technology described by its title. Chang discussed methanol-to-olefin conversion in the presence of molecular sieves at pages 21-26. The examples given by Chang as suitable molecular sieves for converting methanol to olefins are chabazite, erionite, and synthetic zeolite ZK-5.
U.S. Pat. No. 4,328,384 and U.S. Pat. No. 4,547,616 teach conversion of oxygenates to olefins using a fluidized zeolite catalyst. The use of a silicoaluminophosphate molecular sieve to produce light olefins from aliphatic hetero compounds is disclosed in U.S. Pat. No. 4,677,243. U.S. Pat. No. 4,973,792 teaches fluidized catalytic conversion of hetero compounds to light olefins including a purge prior to regeneration of the catalyst. U.S. Pat. No. 6,166,282 teaches oxygenate conversion using a fast-fluidized-bed reactor featuring reduced catalyst inventory compared to earlier processes. The teachings of all of the above patents are incorporated herein by reference thereto.
The art discloses coating of processing equipment in several instances to prevent undesirable side reactions. U.S. Pat. No. 6,548,030 inter alia teaches a low-sulfur catalytic reforming system with at least one surface portion having a protective layer to resist carburization and metal dusting. U.S. Pat. No. 5,406,014 discloses a method for dehydrogenation in which a steel reactor system is provided with a protective layer to resist carburization. U.S. Pat. No. 6,602,483 B2 teaches a hydrocarbon conversion process using steam, exemplified by thermal cracking and ethylbenzene dehydrogenation, in which the steam requirement is reduced by a metal-containing coating on the reactor system. US 2004/0152935 A1 discloses a method for reducing metal-catalyzed byproducts from undesirable methanol conversion in a feed vaporization and introduction system up to the point that methanol enters a methanol-to-olefins reactor by coating heaters, feed lines and feed nozzles. William L Holstein teaches that the presence of water in chemical processes involving methanol maintains iron surfaces in an oxide state which is inactive for the formation of filamentous carbon in IND. ENG. CHEM. RES. 1994, 33, 1363-1372.
The Department of Energy report DOE/ET/14914 of April, 1986, CONVERSION OF METHANOL TO GASOLINE Extended Project: METHANOL TO OLEFINS/Modification and Operations of the Demonstration Plant/MILESTONE REPORT covers a methanol-to-olefins demonstration project using a modified 100 barrel-per-day plant which previously had been used for demonstration of a methanol-to-gasoline [MTG] project. Experimental runs in the demonstration plant showed that at 375° C. carbon steel can catalyze methanol decomposition, necessitating replacement of the superheater with a new one made from stainless steel. U.S. Pat. No. 4,046,190 is drawn to a heat pipe device comprising capillary grooves and metal wicking between the plates, and discloses that “It has been found that copper, brass, nickel and stainless steel are compatible with methanol at 55° F.” The publication, “Effect of Oxidizing and Reducing Gas Atmospheres on the Iron-Catalyzed Formation of Filamentous Carbon from Methanol, IND. ENG. CHEM. RES. 1994, 33, 1367-1372, discusses methanol decomposition and filamentous carbon formation on iron surfaces, recognizing longer induction periods for stainless steel.
The above references acknowledge the issue of metal-catalyzed coking when converting an oxygenate in a fluidized-bed reaction zone, but suggest that the problem may be avoided by the use of water in the process or with stainless-steel equipment. The present invention identifies the unanticipated problem of metal-catalyzed coking under these conditions and offers a solution for protection of the surfaces of a reaction zone.